The present invention relates generally to the field of precision machining and, more particularly, to a simple class of techniques and apparatus therefor which provide automatic sensing and characterization of abnormal operating states of precision machine tools, positioning mechanisms and motion devices.
Precision machining and assembly require very high absolute precision as well as very high positional resolution to obtain significant product yield. In particular, when precision manufacturing is automated, simple and robust techniques are required to provide feedback to the controller about the precise relative positions of the current workpiece surface and, e.g., a machine tool being used to form the workpiece.
The current state of the art in the area of measuring linear and/or angular displacements (or equivalently positions) involves considerable use of feedback systems (encoders, grating scales, laser interferometers, etc.), which produce an electronic output (typically digital counts) proportional to a input motion. Encoders are capable of providing extremely high-resolution feedback on the function of a machine tool. Rotational position is generally measured directly using a rotary encoder, and linear position can be measured using a linear encoder, or by using a rotary encoder connected to the drive of, e.g., a lead screw on a machine tool.
Machine tools are commonly equipped with encoders (usually optical in nature) which provide high-resolution position feedback to an intelligent machine tool controller, which uses this feedback to control the operation or motion of the cutting tool. One type of modern optical encoder, sometimes called a grating-based encoder, functions by detecting the light-to-dark transitions which occur as motion of the encoder input moves precisely placed lines on a reference reticle past a photodetector subassembly. The output is typically a series of pulses which are counted to determine the total magnitude of the displacement. Linear displacements are measured using lines on a sliding reticle, and angular displacements are measured using lines on a rotating reticle. Optical encoders with rotational resolution of 1 arc-second or less are commercially available, as are optical encoders with a linear resolution of 1 micron or less.
Other types of encoders exist which, although they function using different principles, essentially provide the same sort of displacement information as those described above. These would include sine/cosine optical encoders, laser interferometers, inductively triggered encoders, Hall effect encoders, mechanically triggered encoders, and the like. Such encoders are typically either more complex or have lower performance than the grating based optical encoders described above, but are compatible in principle with the present invention.
Unfortunately, there are significant complications in relating the information provided by such encoders to exactly what the machine tool is doing. As an example, an encoder can be used to measure the rotary input to a lead screw on a lathe. The machine tool controller can mathematically convert the encoder output into linear displacement of the tool relative to the workpiece given the pitch of the screw, the known irregularities in that pitch, and such factors as the magnitude of the mechanical backlash of the machine tool drive. However, one cannot take into account such factors as contaminants and particles on the lead screw, backlash which differs (e.g., due to ongoing wear) from that expected from previous characterization of the machine tool, the onset of mechanical defects in the tool, defects in the encoder, its electronics, or its linkage to the machine tool, or complete failure of the feedback controller. This last is perhaps the most important abnormal condition to detect. In addition to piece wastage, such out-of-control function may result in fundamental damage to the machine tool and in a safety hazard as well. Feedback provided only by a simple encoder per motion axis, however, cannot easily be used to alert the machine tool controller to these abnormal conditions.
There is therefore a need for a simple independent reference diagnostic which allows the state of the machine tool to be checked against a standard routinely during operations. Such a diagnostic will insure that an automated machine tool is presently functioning within tolerances suitable for the current workpiece, allows prediction of when maintenance operations will be necessary, allows detection of current operations outside of tolerances, and prevents run-away failure modes.